Obtained during the general course of the main storyline.
This particular achievement is awarded for completing the branch aimed at locating the hive fleet.
The particular mission in which you obtain this focuses on recovering the Astronomic Array, and has you fight Orks and Tyranids, whom are fighting eachother.
In my experience, the easiest way to get through this mission is to use Cyrus with remote detonation explosives, cloak into the middle of each battlefield, plant, run and detonate. Even if detected, the enemy are pretty much tied up in battle to do anything. The few that remain can just be swept up with another squad.
When you get to the Astronomic Array, and have to recover the power coupling, you are given three possible looters. Only one has the coupling, and it may be random, but for me it was the one in the middle. Upon recovering the coupling and activating the Astronomic Array, you are given the choice of going to the extraction zone, or going to another location to learn the location of the Tyranid monster that attacked your squad. Either location ends the mission, so probably best to find the whereabouts of the Tyranid.